The storage of mechanical energy using compressed air has a long history. Air is compressed using a compressor for storage in a tank or reservoir, and when the energy is needed, the compressed air is fed to a motor (e.g. turbine or piston engine), and the mechanical energy is either used directly to perform work or converted to electricity using a generator. Some such systems were developed for powering vehicles, such as trains or cars, and systems have been developed for storage of electrical power where electric motors drive the compressors during storage, while generators are driven by the compressed air motor or turbine for providing the stored power contained in the compressed air storage.
In one example, it has been proposed in recent years to store a large amount of energy, such as tens to hundreds of megawatt hours, in underground salt mine caverns. Such storage can be done without much environmental impact, and is considered to be a “green technology”. An example is found in US patent publication 2011/0127004. The storage of tens to hundreds of megawatt hours (MWh) is beneficial to electrical power generation and distribution networks, where supply and demand do not always coincide.
As is well known, when air is compressed, its temperature rises. The amount of heat released by compressing air stored at ambient temperature increases with the pressure of storage. In an underground cavern, the preferred pressure for storage is about 65 to 85 bars, and the temperature of the compressed air can be around 250° C. to 750° C. In US patent publication 2011/0127004, it is known to use thermal storage for storing the heat of the compressed air before sending it to the storage reservoir, and to restore heat to the compressed air before using it in an air motor-generator.
It is recognized in 2011/0127004 that the construction of a heat exchanger that can handle pressures between 65 and 85 bars is a challenge. For this reason, some prior art systems are designed to simply cooling of compressed air and heating of restored air at the sacrifice of efficiency. For example, some systems cool compressed air quickly using a water spray (see patent application publication US20120286522). Other systems use the heat of combustion to restore heat during recovery (see U.S. Pat. No. 5,491,969).
The problems associated with known CAES systems, as for example are described in US patent publication 2011/0127004, include the need for a large storage reservoir for the compressed air and either the size or high cost of the heat exchanger required. When the heat exchange system is simplified, this can come at the cost of heat losses that reduce efficiency of the CAES system.